1. Field of the Invention
The present invention is in the field of sensors. That is, the present invention relates to sensors which provide an electrical output signal in response to an input of another type. The input may be in the form of light, or other electromagnetic radiation. The electrical output response of the sensor may be used to provide an indication of the existence of an input source, of the direction of the source relative to the sensor, or to image the source, for example.
Such a sensor according to the invention further includes a self-adapting sensing circuit. A feature of the self-adapting sensing circuit allows the sensor to operate in any one of several modes. That is, in the operation of such sensors, it is frequently desirable on the one hand to compare the signal provided by a sensor element of the sensor to either an average of the signals provided by the sensor as a whole, or by the particular sensor element over a period of time. Thus, the output signal provided by the sensor is "time averaged", and features of the input source which change with a time interval in a range shorter than a certain maximum interval and longer than a determined minimum interval, each associated with a certain minimum and maximum response frequency for the sensor, are detected. That is, input source features which are changing with time within a selected range of frequencies or time intervals of change, are detected by the sensor.
On the other hand, it may be desirable to compare the output signal of a sensor element to its own immediately preceding output signal in order to provide an output signal indicative of the difference in these signals. This comparison and output signal provision effectively provides a pseudo-radiometric output for a sensor responding to photons of light. The output signal of the sensor is indicative of changes in the input source without reference to the frequency, time interval, or time rate of change of the level or value of the input source, but with reference to a predetermined (known or unknown) value.
Alternatively, it may be desirable to provide such a sensor with an output which is referenced to an externally-provided value. That is, in the case of sensors responding to infrared light, it may be desirable to reference the output of the sensor to a level of infrared photons, such as from a black body source at a set temperature, for example. Such a sensor provides a calibrated output which is radiometric in the sense that the output signal from the sensor is proportionate to the incident infrared light flux according to a proportionality based on the temperature of the reference black body source.
Still more particularly, the present invention relates to such a sensor which is photoconductively responsive to infrared light, and which includes an array of individual sensor elements which are individually adaptive. The sensor may be either of a linear-array or of a two-dimensional array type, and may be a fully staring sensor which does not require a chopper or any other moving parts, for example.
2. Related Technology
Conventional photoconductive sensors generally include an array of sensor elements, each of which provides its own individual response to the particular portion of the input infrared light which falls upon a particular sensor element. The individual electrical outputs of the sensor elements are conducted outwardly of the sensor in order to provide an indication of existence, direction, or image information for the source of infrared light, for example. The sensor may include either a linear array of such sensor elements, or a two-dimensional array of sensor elements. When a linear array of sensor elements is used, the usual sensor system includes a scanner or other such device to scan various parts of the input across the linear array so that all parts of the "scene" (or field of interest within which input sources may be found), scan across the linear array of sensor elements. Alternatively, relative movement of the sensor or source may be used to effect relative movement of portions of the "scene" across the sensor.
Further, conventional focal plane array imaging devices (both for visible light, and for other portions of the electromagnetic spectrum, such as for the invisible infrared portion of the spectrum) have been known for some time. These devices are generally of the charge-coupled type or of the direct-injection type. For purposes of convenience and simplicity in description hereinafter, the term "light" or "light-responsive", and other such terms, should be understood to refer to the electromagnetic spectrum in general, and may include both infrared, and ultra-violet radiations, and other wavelengths in addition to visible light. The known conventional focal plane array imaging devices currently are fabricated as arrays of light-responsive elements, or pixels, in the form of thin-film devices generally in a rectangular array of photo-responsive receptors on the face of a semiconductor substrate. The devices are fabricated using conventional CMOS, thin-film, and other currently-known semiconductor fabrication techniques.
Other conventional infrared or thermal imagers use "room temperature" or near room temperature, ferrielectric sensors, which may be fabricated of barium strontium titanate (BST), for example. Another conventional infrared sensor uses a thin-film bolometer fabricated as a current-mode monolithic array. Such BST or bolometer sensors are considerably more expensive to make than are photoconductive infrared sensors. However, prior to the present invention, photoconductive infrared sensors could not generally provide a level of sensitivity and performance favorably comparable to sensors using the more expensive technologies. Circuit techniques to improve the performance of the photoconductive sensors could not be implemented at a small enough size to be packaged with the sensor in a thermal enclosure. If the performance enhancing circuits were implemented outside of the thermal enclosure, a large number of conductors were required to penetrate the thermal enclosure between ambient and the chilled sensor. Each of these electrical conductors also represented a thermal conductor which allowed ambient heat to leak into the thermal enclosure. The cooling requirements of such a sensor could either rule out the possibility of properly cooling the sensor to its optimum response temperature, or would require a prohibitively large expenditure of power to achieve this level of cooling despite the comparatively large heat leakage into the thermal enclosure along the multitude of conductors.
Importantly, known conventional imaging devices of the focal plane array type are based on an architecture which requires the pixels of the device to be accessed in serial order. That is, the image signal from the pixels is fed out of the imaging device as an analogue or digital data stream representing light levels incident on the pixels individually in a row-by-row scan of the array. Generally this scan starts at one corner of the rectangular array and proceeds across the row of pixels individually, preceding subsequently across the next or adjacent row of pixels. Of course, scanning every other row of the array with the scan rate being such that two such partial scans of alternate rows are completed in the same time as would be required for a complete scan of adjacent rows is also known to reduce the flicker of a video image (interlacing). With either type of scanning, this type of serial image output signal indicative of a pixel scan is long-familiar from the television technology.
Unfortunately, when it is desired to foveate, or to concentrate attention on a stationary or moving image which resides in a particular part of the image array and occupies a comparatively small portion of the array, a large part of the serial information in the signal stream is of little or no interest. That is, after the last portion of the serial signal stream which includes information about an image of interest is received, almost the entire remaining portion of the array scan (or scan of interlacing alternate rows) must be completed before the scan will return to the area of the array which is of particular interest. Thus, time is lost in acquiring image information from the part of the array which is of most interest. This time loss is the case even is signal acquisition circuitry is employed to acquire and concentrate attention on (i.e., create a window of image out of) the array signal stream.
Similarly, when it is desired to acquire an image of an image source which is fast-moving, for example, then the time lost in scanning the entire array, including those areas of the array where image information of little or no interest is located, is a great detriment. This time loss can result, for example, in loss of the image source from the field of view of the imaging system, in confusion of background noise sources for the image source of interest, or both.
One conventional expedient is to simply increase the scanning rate at which the pixels of an imaging device are accessed. This increase of scanning rate results in the scan returning to the area of the array which is of interest more quickly and with less loss of time between scans of the interesting area of the sensor array. However, when the image to be generated is digitized, the analogue-to-digital converters (digitizers) themselves have a finite settling time which limits the rate at which the array can be scanned. The conventional solution to this lack of speed in array scanning is to use plural digitizers in parallel. In this architecture, each of the plural digitizers in sequence is supplied with a portion of the analogue data stream, and is then interrogated for its portion of the resulting digital signal after the digitizer has had time to settle. With multiple digitizers sharing the load, doubling the scanning speed requires double the number of digitizers. Of course, it is easily seen that this conventional expedient itself has limitations with respect to the cost and complexity of the overall imaging system. As the rate of pixel scanning increases, the number of digitizers required becomes prohibitive.
Another conventional expedient is known in accord with U.S. Pat. No. 5,262,871, issued 16 November 1993, to Joseph Wilder, et al. The '871 patent is believed to disclose a two-dimensional focal plane array sensor in which individual pixels or super pixels (groups of individual pixels) of the array can be accessed individually for their output response. This teaching allows the output signals of pixels on the array which are of interest to be obtained, which skipping the outputs of pixels having output (image) information which is of lesser or of no interest at a particular time. Understandably, the time required to complete a scan of the array is significantly shorter when only part of the pixels are interrogated for their output information, when pixels are grouped into super pixels for which averaged output information is obtained, or both.
The '871 patent illustrates another aspect of conventional technology, and an aspect which limits the utility of this technology. In order to provide the electrical output signals from an array of sensor elements to external signal processing circuitry, the sensor taught by Wilder uses a "signal read out section" (or multiplexer) to provide a serial stream of output signals to the external circuitry for further processing. The pixels of the array use a photodiode type of photon sensor which is actually the sensor element of each pixel. These pixels are accessed individually or in groups in order to obtain their individual or a group-average output signal. However, the output signals themselves are received by the "signal read out section", or output multiplexer via an input port or connection which receives a serial stream of output signals from the selected sequential ones or from all of the pixels in a particular row of the sensor sequentially, for example. The input port does not discriminate between individual pixels of the multiple pixels served by a particular signal input port, other than to attribute the input signal received to a particular pixel whose address has been interrogated. So far as adapting the signal input port for individual differences in the pixels of the array, the '871 device does not have this capability. Also, the device of the '871 patent does not provide for individually different processing of input signals from the pixels served by an input port and which are received sequentially. Accordingly, all of the pixels from which an output signal is provided into the output multiplexer via a particular port or connection, and in fact, all of the pixels of the sensor, are treated equally and as equals so far as signal processing (i.e., application of a bias current level or voltage, for example) is concerned. Accordingly, the sensor of the '871 patent cannot implement the alternative modes of signal processing outlined above.